The invention relates to a method for determining a gas concentration in a flowing, solids-containing gas mixture, and also to a device for carrying out such a method.
In the chemical industry it is frequently necessary, e.g., for process control, to determine the concentration of a gas in a solids-gas mixture. This proceeds, for example, toward the end of a process, in order to determine the fraction of a gas in a solids-containing product and starting material mixture before the product is separated from the unreacted residual starting materials or any byproducts and removed from a reactor. Likewise, the gas fraction can be determined during a reaction, in order, for example, to determine the fraction of a starting material gas or the fraction of a product gas in the reaction gas mixture.
Gas analyses of this type are also known from the fired-plant industry, since the operators of such systems are legally required to regularly demonstrate compliance with emission limiting values. In the case of measurements of the fractions of CO, NO, NO2, SO2, etc. in flue gas, for example paramagnetic oxygen measurement and nondispersive IR spectroscopy are used.
Particular attention is given in measuring technology to the determination of the gas concentration of oxygen. The importance of oxygen lies, in particular, in the control of a combustion process, monitoring of a reaction, or in safety aspects.
From the automobile industry, the exact oxygen measurement in exhaust gas is known using what is termed the lambda probe, in order to control the fuel-air mixture. Here, the fact that zirconium dioxide can transport oxygen ions electrolytically at high temperature is utilized, whereby a voltage is produced.
For oxygen measurements, therefore ZrO2 probes come into consideration. This measurement is based on the fact that in a measurement cell a reference gas (e.g., air) is separated from the sample gas by a zirconium oxide membrane which is coated on both sides with platinum. An electrochemical cell is formed thereby, which, in the event of a difference in the oxygen concentrations on both sides, leads to an oxygen gradient over the thickness of the zirconium oxide membrane and to an electrical potential difference between the platinum electrodes. From the voltage drop, the oxygen partial pressure may be determined.
During the monitoring of heating boilers, a lambda probe can measure the oxygen content of the exhaust gas, and thus control at the boiler an optimum mixture in order to prevent an excess or deficit of combustion air.
However, in applications in gas streams having a high particle fraction and possibly substances which cause a cross sensitivity or give rise to aging of the probe, these probes are too susceptible and the plants operated with such probes are very maintenance-intensive.
A further possibility for oxygen measurement is diode laser spectrometers. A detector measures the absorption of the laser light by the gas molecules. The gas concentration may be calculated therefrom.
However, the use of a diode laser spectrometer is always a problem if a process gas is to be studied that has a comparatively high solids fraction. The solids impair the transmission of the laser and thereby the measurement result.
Therefore, in the prior art, efforts have been taken to improve the concentration measurement in gases loaded with solid.
In CN 100545634C, for this purpose, the use of displacement bodies or blocking systems is proposed, in order to deflect or block the solid particles. The displacement bodies have, for example, the shape of baffle plates. However, it has been found that in dilute solids-gas streams and certain highly fluid solids, owing to vortex formation and reverse streams, solids can pass onto and behind the baffle plates, and here also the laser beam is impaired.
Conventional laser spectrometers can only be operated reliably in the case of gas mixtures having a particle fraction up to approximately 50 g/m3.
However, in the chemical industry there are applications in which the particle fraction is markedly higher.
One example thereof is process control in the production of highly dispersed silica.
In the production of polycrystalline silicon by deposition from chlorosilanes and hydrogen, e.g. in a Siemens reactor, silicon tetrachloride (SiCl4) arises.
The production of SiO2 powders (highly dispersed silica) via flame hydrolysis is known, for example, from DE2620737 and EP790213. In addition to the abovementioned silicon tetrachloride, a multiplicity of other silicon-containing compounds and mixtures thereof can also serve as feedstocks, e.g., methyltrichlorosilane, trichlorosilane or mixtures thereof with silicon tetrachloride. Chlorine-free silanes or siloxanes can also be used. According to EP790213, the use of dimeric chlorosilanes and siloxanes is also possible.
For process control and safeguarding, oxygen measurements are required in the production of silica.
In the production of silica, the gas stream usually comprises a particle fraction of greater than 50 g/m3, usually 100-200 g/m3.
This means that without additional devices, measurement of gas concentrations by way of conventional optical measurement technology, such as, for example, oxygen measurement by way of laser technology, is not possible.
The displacement bodies proposed in CN 100545634C, when employed in the production of silica produced by flame hydrolysis, lead to uncertain measurement results. This is associated, firstly, with the size and density of the particles present in the gas stream. Also the extremely low inertia of the particles has proved to be disadvantageous for the measurement. The laser measurement is impaired by the displacement bodies, owing to vortex formation, not being able to ensure that no particles pass into the measurement section. The transmission of the laser is too low in such a gas stream. Especially in the production of silica by means of flame hydrolysis, lasers, having too low a transmission value, are not permitted by the technical inspection agency as a protective appliance against the formation of ignitable gas mixtures.
The object of the present invention resulted from these problems.